How deep you dig into your pocket if you were asked to pay only as much as you wanted for a cup of coffee?

An inner-city barista brewed a scheme this week to let customers decide on the price of a coffee and was stunned when his profits went up.

The innovative marketing antic has exposed a little-mentioned side to the Kiwi consumer psyche - if offered a chance to set the price we're likely to pay more than the usual cost.

Dr Douglas Elliffe, head of psychology at Auckland University, said an "embarrassment factor" was likely to come into play.

"No one likes to look mean in public or be seen to be ripping someone off," he explained. "It's a bit like when Kiwis who aren't used to tipping in restaurants at home start over-tipping when abroad because tipping is expected overseas and they don't want to look tight."

However, Elliffe doubted similar "pay-what-you-like" schemes would be workable. "If this kind of promotion became permanent I suspect people would soon become increasingly less worried about what others might think and would start paying less pretty quickly."

The theories were put to the people when the ME Coffee Boutique in Auckland's CBD turned three years old on Wednesday.

To celebrate, co-owner Hans Pronk dumped his price list and allowed customers to choose how much they wanted to pay for a coffee. "The lowest someone paid was 10c and I also got some euros and Australian cash," Pronk said.

"But a few people gave $20 and told me to keep the change."

It usually costs $3.50 for a long black at ME. A flat white is $4.20 and a large cup $4.70.

Pronk said plenty of regulars paid about $5 on the day. He sells as many as 300 coffees a day.

"I was expecting to take a hit at the till but my takings were actually slightly up. Maybe people are more generous than you might think, but I don't have the bottle to try the idea in the long run."

Some cheeky customers bartered. One offered a packet of instant coffee in exchange for the real thing. Another handed over a string of 12 duck sausages for a mochachino.

Dollar - or cents

Workmates Matthew Hamilton and Vanessa Wintle raised a cup to pay-what-you-like day at their favourite coffee bar.

The stenographers from the Auckland District Court each paid wildly differing amounts. They usually visit the nearby ME in Albert St twice a day.

Wintle, 28, handed over $5 for a regular flat white (usual price $4.20) and Hamilton, 30, parted with just 20c for the same because he helped decorate the shop for its third birthday. "I was happy to pay a bit over the odds as a gesture of good will because the quality of the coffee and the social side of the place is so good," Wintle said. "To be honest, I didn't want to look tight-fisted either."